Supercapacitors (also known as ultracapacitors) have been attracting numerous interests because they can instantaneously provide higher power density compared to batteries and higher energy density compared to the conventional dielectric capacitors. Such outstanding properties make them excellent candidates for applications in hybrid electric vehicles, computers, mobile electric devices and other technologies.
Generally, an electrochemical capacitor may be operated based on the electrochemical double-layer capacitance (EDLC) formed along an electrode/electrolyte interface, or a pseudocapacitance resulted from a fast reversible Faradaic process of material that undergoes Faradaic reactions (a “Faradaic material,” e.g., redox-active materials such as metal oxides and conductive polymers). In the present application, an EDLC-based capacitor is referred to as a double layer supercapacitor (DLS) and an electrode material coated onto a current collector in a DLS is referred to as a DLS material; a pseudocapacitance-based capacitor and/or one based on ion insertion is referred to as an electrochemical supercapacitor (ECS) and an electrode material coated onto a current collector in an ECS is referred to as an ECS material; an electrode material coated onto a current collector in a battery (e.g., Galvanic cell) is referred to as a battery material; “electrolyte” refers to the material which provides the ionic conductivity between supercapacitor electrodes; and “charge collector” refers to an electrically conducting material that connects the supercapacitor to an electronic circuit or other device(s).
For a DLS, the rapid charge/discharge process provides the capacitor with a high power density, yet the energy density is limited by its effective double-layer area. To date, a large number of DLS materials (e.g., high-surface-area materials, such as activated carbon, templated carbon, and carbon nanotubes (CNTs)) have been extensively studied. Activated carbons, with surface areas from 1000-2500 m2/g, are the most commonly used materials, which may provide a capacitance up to 320 F/g at low potential scanning rate. However, the capacitance may drop dramatically at high scanning rates because of their tortuous pore structure and high microporosity. The templated carbons, on the other hand, exhibit uniform pore geometry and larger pore size; however, they did not show any exciting improvement in either energy or power performance. For comparison, multi-walled CNTs show capacitances up to 135 F/g and single-wall CNTs show capacitances up to 180 F/g, which are still low for an actual device application.
Compared with the DLS materials, ECS materials (e.g., based on metal oxides or conducting polymers) may provide much higher specific capacitances (e.g., up to one thousand farads per gram of ECS material). However, actual applications of ECS are still limited by high cost, low operation voltage, or poor rate capability, mostly because of inefficient mass transport or of slow faradic redox kinetics. Specifically, such high electrical resistance can limit the practical thickness (smallest dimension) of oxide electrodes, as increased thickness leads to increased electrode resistance, reduced charge transport and/or low power.
Consequently, in spite of extensive research and effort, making supercapacitors with high energy and power density still remains challenging. Supercapacitor electrodes of the prior art have not provided the device performance (e.g., energy density, power density, cycling stability, operating voltage) and manufacturability required for many high-performance, commercial applications.